How to make a simple Cold Frame


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Here is the promised article on how to make a cold frame. This is a simple project that should not take more than 2 – 3 hours. I built both of these in 3 hours, the longest part of the job is measuring and cutting.

Standard pine shelving was used for the cold frame
Standard pine shelving was used for the cold frame
Cold Frame pieces cut to size
Cold Frame pieces cut to size

A cold frame is used specifically to allow for the germination of seeds and protection of seedling in sub-optimal weather conditions. Typically the cold fame is closed just before sunset so that the frame can store heat at night and opened soon after sun-rise, to ensure that the seeds or seedlings are not cooked during the day. This is very important and will need to be controlled as weather conditions change. We typically will have 3 lengths of wood that prop open the lid depending on what temperatures are experiences. During hot weather you will want to have the lid open on maximum and vice-versa for colder weather.

Watering is also important as the seed trays and pots used inside the cold farm will require at least an everyday watering and sometimes more, you will need to maintain a watch and if you see wilting use a watering can with a fine rose to water your seedlings.

Ok onto the cold frame. We used standard pine shelving (2400 x455 mm wide) from our local board centre. I bought 4 pieces but only needed 3, to make two cold frames. The wife is happy as she gets a new shelf in her pantry that she has been pleading for with the 4th piece.

I’m going to give rough dimensions as each person will have their own size requirements, and the two that I made will allow us to germinate over 4000 seedlings in one go. There are not many gardeners out there that need to germinate so many seedlings, unless you are supplying seedlings at a farmers market. So you can adapt this to your needs.

A few screws and some cold glue and the basic frame is completed. Note the handles made from scrap wood on the sides.
A few screws and some cold glue and the basic frame is completed. Note the handles made from scrap wood on the sides.

Basically the design will work on the following principles. The back of the cold frame (South side) needs to be higher than the front (North side) ours is 300mm at the back and 150mm in the front. We cut one of the shelves into two, lengthways to give us the 300mm high back and the 150 mm front pieces. The sides of this cold frame were cut from a full width board and we measured to the same height as the front and back boards.

These four pieces were screwed together with standard 40mm chipboard screws using a pilot hole to prevent the board ends splitting. Handles were cut and screwed on made from off cuts, this allows two people to pick up a cold frame and move it easily.

For the lid of the cold frame we used Thermoclear but you could use any rigid transparent material. We decided to use Thermoclear as a) we had a spare sheet lying around and b) it has a twin wall with wonderful rigid cells running the length of the sheet. It’s the perfect way to make double glazing with minimal effort. Although the boxes are a bit draughty, I felt that this little bit of extra insulation should keep the heat in and cold out for a while longer. When measuring your lid, have the material overlap by 15 – 20 mm all around as this will allow moisture to drip away from your wood and helps to preserve the cold frame.

Edge on view of the Thermoclear showing it being filled with silicone.
Edge on view of the Thermoclear showing it being filled with silicone.

This plastic sheeting is also great, because if you carefully cut through the top layer of plastic you have a hinge and it solves a lot of construction issues with additional hinges and effort. We sealed the ends with clear silicone to create a ‘double glazing’ effect on the lids. Just make sure you dry the silicone in the shade, as it’s amazing how quickly the heat (pressure) builds up in each tube and starts to push the silicone out before it has a chance to dry.

One everything is dry, oil the wood to keep it weather proof, we use Waksol (R90.00 for 5 Lt) but use up whatever you have lying in your workshop/garage.

Close-up of the simple 'hinge'. The top layer of theroclear has been cut to 'create' the hinge.
Close-up of the simple 'hinge'. The top layer of thermoclear has been cut to 'create' the hinge.

The cold frame is then placed directly on a tarp which is laid on level ground, the seed trays and pots (for cucurbits and larger faster growing seeds) are then placed inside onto the tarp. The tarp is there for two reasons, 1) to prevent grass and weeds coming up through your seed trays and 2) it helps to keep the roots inside the cell divisions / pots. If you place the trays or pots directly on the soil then the seedlings roots grow straight into the soil and you destroy half of the plants root structure when you lift them to transplant. In larger more formal seedling operations the seed trays are placed onto a wire frame to discourage the roots from growing through, we are not quite there yet.

Job done, all it needs now is a layer of preservative to keep it looking good for years.
Close-up of the simple 'hinge'. The top layer of theroclear has been cut to 'create' the hinge.

That’s about it for making a cold frame, it’s simple and easy, and will allow you to get a good 5 – 6 weeks head start on the planting season.

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